An airbag is known as a safety system for protecting a passenger from an impact resulting from a collision of an automobile. The airbag is activated by a large quantity of a high-temperature and high-pressure gas generated by a gas generator. Conventionally, gas is generated by gas generators according to two roughly-categorized methods. One is a pyro inflator method in which all gases are generated by burning solid gas generants (for example, refer to Patent Document 1). The other is a hybrid inflator method in which a gas cylinder containing a high pressure gas and a small quantity of explosive compositions for supplying heat to a high pressure gas in the gas cylinder are used to discharge a large quantity of a high pressure gas (for example, refer to Patent Document 4).
Where gas generant compositions include low reactive materials such as guanidine derivatives and ammonium nitrate as its compositions, there poses a problem that the gas generants are low in ignitability, in addition to a slow burning velocity. An airbag takes 10 to 60 ms from the time when a gas generator is activated to the time when the airbag is deployed. Therefore, only a slight delay in activation of the gas generator will give such a serious impact that the airbag cannot exhibit a sufficient performance. Gas generants, which are low in ignitability, will take a longer time to ignite on a squib inside the gas generator, resulting in a delayed ignition of the gas generator. An increased quantity of a propellant inside the gas generator will improve the delayed ignition to some extent. However, the thus increased quantity of the propellant will lead to an increase in total calorific value of the gas generator itself, which consequently increases the weight of a filter member for cooling the gas and makes the gas generator larger in size.
Further, a gas generator using the pyro inflator method for inflating an airbag for side impact and a curtain airbag is mainly an elongated cylindrical housing, as disclosed in Patent Document 2, and structured in such a manner that both ends are sealed by welding. Therefore, where the gas generant disclosed in Patent Document 1 is used, the gas generator must burn the gas generant at a higher pressure in order to obtain a sufficient burning performance, and a housing must be thicker in order to improve the pressure resistance of the housing. Where a propellant is used in a larger quantity to improve the ignitability, a filter member must be larger to give a sufficient cooling to the gas, because of an increased calorific value. Under these circumstances, conventional gas generators have not been made smaller in size or lighter in weight.
Further, Patent Document 3 has disclosed a gas generator according to the pyro inflator method in which a seal member such as an O-ring is provided on an outer peripheral end surface of a partition member in order to seal reliably a housing and the partition member. However, this gas generator needs a process for notching the outer peripheral end surface of the partition member to provide the seal member such as the O-ring, thereby increasing a cost of manufacturing the gas generator, which poses a problem. In addition, in the gas generator, the partition member does not divide a filter chamber and a combustion chamber.
In contrast, a gas generator according to the hybrid inflator method has a smaller quantity of gas generants and is suitable for making the generator small in size. However, since the gas generator must keep a high pressure gas in a gas cylinder, the high pressure gas may escape from the gas cylinder in the course of as long as 15-year life period as a gas generator, thereby resulting in a failure of providing a sufficient effect, where sealing is insufficient. For this reason, for sealing the gas in the gas cylinder for a longer time, which makes it necessary to seal the gas cylinder by using higher in breaking strength and a highly-sealable rupture disk. This type of gas generator is disclosed in Patent Document 4, for example. In order to increase the gas sealability of a first container (gas cylinder) in which a high pressure gas is sealed, a rupture diaphragm (rupture disk) having a high breaking strength is used in the gas generator, and a hollow piston provided in a second container having a combustion chamber, etc., is allowed to advance into the rupture diaphragm by ignition of a driving insert (squib), thereby rupturing a rupture diaphragm of the first container without fail and also reliably discharging a high pressure gas in the first container. This structure is able to rupture the rupture diaphragm without fail but requires a hollow piston or the like, thereby making the gas generator structurally complicated, which also poses a problem.
There is also available a gas generator in which rupture of a rupture disk for reliably sealing a gas cylinder is conducted not by using a hollow piston or the like as disclosed in Patent Document 4 but by increasing a quantity of a propellant to elevate the pressure inside a combustion chamber including a squib. However, an increased quantity of the propellant requires a chamber for accommodating the propellant, thereby making it difficult to miniaturize the gas generator. Further, in this instance, it is difficult to rupture the rupture disk simultaneously with ignition of the squib.
Further, in this type of gas generator, a high pressure gas inside a gas cylinder undergoes an adiabatic expansion to spout from the gas cylinder, thereby making it necessary to heat and discharge the gas. This generator is unable to sufficiently heat the gas from the gas cylinder, which poses another problem.
Still further, Patent Document 5 has disclosed a gas generator according to the hybrid inflator method in which a gas cylinder for retaining a high pressure gas and a small quantity of explosive compositions for supplying heat to the high pressure gas inside the gas cylinder are used to discharge a great quantity of a high temperature and high pressure gas.
In addition, Patent Document 6 has disclosed a squib used in these gas generators in which a thin-film bridge formed on a substrate of ceramics or the like is fixed to a plug by using epoxy resin, polyimide, ceramics and others. This thin-film bridge is electrically connected to an electrode pin provided on a plug by soldering or wire bonding or by using an electric-conductive epoxy resin or the like. A squib having the thin-film bridge is advantageous in that the ignition time is reduced to several times or more than a dozen times, as compared with that having a bridge wire. However, where the squib is connected with an electric-conductive epoxy resin and used in a gas generator for an automobile, it is exposed to a high-temperature environment on blazingly hot days in summer, which may result in a change in resistance value of the electric-conductive epoxy resin. Further, the resistance value is easily affected by the surface condition of an electrode even at an initial stage of assembly, resulting in a great variance in initial resistance value, which poses a problem. Wire bonding will solve the problems caused by soldering or by the use of an electric conductive epoxy resin, however, in the squib disclosed in Patent Document 6, a thin-film bridge is fixed in a state that it is projected on a plug. Therefore, a wire may be broken where a pressing force is applied upon insertion of the plug having the projected thin-film bridge into an ignition-charge loading cap. The possibility may be higher particularly where, as disclosed in Patent Document 6, the wire is erected to joint on the end surface, which is a so-called vertical joint.
[Patent Document 1] Japanese Unexamined Patent Published No. 11-292678
[Patent Document 2] Japanese Unexamined Patent Published No. 2000-203372
[Patent Document 3] International Publication WO No. 01/74633, booklet
[Patent Document 4] Japanese Unexamined Patent Published No. 8-253100
[Patent Document 5] International Publication WO No. 02/062629, booklet
[Patent Document 6] U.S. Pat. No. 6,324,979B1 Specification